Habitat damage Bottom trawling by fishermen may be even more damaging than previously thought, affecting the seabed as seriously as intensive ploughing of farmland erodes the soil, say Spanish scientists.

Bottom trawling - dragging nets across the sea floor to scoop up fish - stirs up the sediment lying on the seabed, displaces or harms some marine species, causes pollutants to mix into plankton and move into the food chain and creates harmful algae blooms or oxygen-deficient dead zones.

"Bottom trawling has been compared to forest clear-cutting, although our results suggest that a better comparison might be intensive agricultural activities," they write in a study published on the journal Nature.

During the 20th century, more intensive farming techniques and changes in land use reduced the diversity of landscapes almost everywhere, say the researchers.

Ploughing up land exposes the top soil to erosion by wind and water, destroying or weakening nutrients in the soil which are essential for many plant species to survive.

As with soil, the seabed is composed of layers of sediment, holding nutrients that are vital for marine life.

While farmers usually plough their land a few times a year, sea trawling can occur on a near daily basis, the scientists say.

Fishing has also become increasingly industrial. As technology has improved and traditional fish stocks have been depleted, trawling fleets have gone into ever deeper waters in search of fish.

Submarine landscape

The scientists measured the movement of sediments on the sea floor caused by fishing activities in a submarine canyon in the northwest Mediterranean Sea.

Deep-sea trawling became fully industrialised in the region in the 1960s and 180 large bottom trawlers currently operate to depths of 800 metres or more.

The scientists found heavy fishing equipment moves sediments on upper continental slopes - the transitions between shallow continental shelves and deep basins - modifying the submarine landscape over large areas.

They linked daily sediment movement to the passage of the trawling fleet, and found some of the movement was similar in size to the sediment transport caused by winter storms in nearby submarine canyons.

Using satellite navigation tracks from bottom trawlers operating in the area, the scientists discovered the tracks coincided with smoothed parts of the canyon at depths shallower than 800 metres. Untrawled parts of the canyon, by contrast, were dominated by a network of valleys.

"The frequent repeated trawling (ploughing) over the same ground, involving displacement of sediments owing to mechanical redistribution, ultimately causes the levelling of the surface and produces morphological effects similar to those of a [ploughed] farmer's field," say the scientists.

The ecological impact of trawling and its influence on changes to the submarine landscape should be considered a danger to the ocean ecosystem alongside global warming, rising sea levels, acidification and changes in ocean circulation, they add.